DS Sr did a strange thing

My system uses three DS Seniors. The volume was set at 70 on all three with the screens blanked. I got up to do something and as I sat down one of the dacs had its screen alight and the volume number was rapidly cycling up and stopped at 70. The remote control was on my settee touching something else, but, I can’t think of anything that could cause the volume to reduce and then need to be counted up and know it must stop at 70. All very strange, has anyone ever seen that?